1. Field of the Invention
This invention relates to methods and compositions for the treatment of submerged surfaces, such as marine or other aquatic structures of ship hulls, in order to prevent fouling of the surfaces by aquatic organisms.
2. Description of the Background
It is well-known that the growth of organisms (micro- or macro-fouling matters) on the submerged parts of a structure may have detrimental effects on their operation and their corrosion rate. For example, in the techniques of oil production at sea, the fouling may accelerate the corrosion of submerged structures such as supports of drilling platforms. The weight increase resulting from the deposit of the fouling matter also results in difficulties when raising up certain submerged structures, as it is the case for the pipe-lines used to collect oil at sea; it also requires frequent operations for the maintenance of the floats of signal or meterologic buoys. On the other hand, the formation of even a very thin layer of microfouling is sufficient to reduce the transmission of light and sound and consequently to disturb the operation of certain devices such as sonar sea-marks. The fouling matter may also be a medium favorable to the proliferation of certain microorganisms responsible for the biodegradation of organic materials and of concrete. It is also known that the cooling systems for plants and power stations, either of the nuclear or of the conventional type, that are operated with sea water are also subject to severe fouling which may plug ducts and condensors. Finally, fouling by large organisms such as the balani, the serpulae and the algae, increases the roughness of the hull of ships and their drag in water, thereby resulting in an increase of fuel consumption and/or a reduction of the ship speed. These various problems and their consequences emphasize the importance of anti-fouling substances.
Besides the periodic cleaning of the surfaces or the use of paints enabling a controlled exfoliation, which are very expensive remedies, the principle of most anti-fouling action is to create a toxic zone on the surfaces to be protected. For example, chlorine is used successfully in a continuous manner in sea water ducts, but this technique is obviously unsatisfactory as far as the preservation of the natural environment is concerned.
An efficient way of combating fouling must (by prior art) comprise the maintenance of the toxic product at an efficient and homogeneous concentration and in a continuous manner on the whole surface. This is the reason why the so-called "anti-fouling" paints have taken an important place among the anti-fouling means. Thus, in order to fight against the growth of sea organisms on submerged surfaces and hulls of boats, an anti-fouling paint is generally applied as an upper layer. According to the known techniques, this anti-fouling paint contains a toxic substance which slowly reacts with sea-water to give a salt soluble in water and which is lixiviated from the paint pellicle. Among the toxic substances which are the most commonly used, there can be mentioned cuprous oxide, tin tri-n-butyl oxide, tin tri-n-butyl fluoride and tin tri-n-butyl sulfide, these compounds being biocidic agents with activity against a wide range of a sea organisms. However, the lixiviation process cannot, even in these cases, be controlled uniformly. Generally, it is much too fast immediately after a submerged structure has been put into service with, as a consequence, the initial presence, near the material to be protected, of very high concentrations of toxic matters, higher than those actually required. This results in a loss of toxic agent and in pollution of the environment and, thereafter, the presence of a lower concentration of toxic material than that necessary to obtain efficient protection, which results accordingly in the accumulation of fouling-organisms.
In order to obviate these disadvantages, a recently proposed solution of the prior art consisted of applying a surface coating containing a polymer wherein the toxic groups are chemically combined, thereby decreasing, as a general rule, the lixiviation rate of the biocidic compound in the aqueous phase and extending the effective life of the paint. These biocidic coatings generally contain organo tin compounds chemically bonded to the polymer substrate through a hydrolyzable ionic bond. They consist essentially of polyesters or polyepoxide resins containing organo-tin derivatives or metal salts, as proposed for example, in French Patent Specification Nos. 2,266,733 and 2,307,957 and U.S. Pat. Nos. 3,167,473; 3,684,752 and 3,979,354. These organometallic resins are generally obtained either by polymerization or copolymerization of organometallic unsaturated monomers, or by reaction of a suitable organometallic compound with a resin comprising carboxyl groups. These methods have been described, for example, in U.S. Pat. No. 3,016,369 and in Journal of Polymer Science vol. 32 No. 125 (1958), pages 523 to 525. However there has been no solution proposed to the problem of fouling of submerged surfaces that does not involve the release of toxic compounds, such as are now in wide use. Moreover, no solution has been proposed for replacing them with substances which cause less harm to the environment.